Continuous web printing allows economical, high-speed, high-volume print reproduction. In this type of printing, a continuous web of paper or other substrate material is fed past one or more printing subsystems that form images by applying one or more colorants onto the substrate surface. In a conventional web-fed rotary press, for example, a web substrate is fed through one or more impression cylinders that perform contact printing, transferring ink from an imaging roller onto the web in a continuous manner.
Proper registration of the substrate to the printing device is of considerable importance in print reproduction, particularly where multiple colors are used in four-color printing and similar applications. Conventional web transport systems in today's commercial offset printers address the problem of web registration with high-precision alignment of machine elements. Typical of conventional web handling subsystems are heavy frame structures, precision-designed components, and complex and costly alignment procedures for precisely adjusting substrate transport between components and subsystems.
The problem of maintaining precise and repeatable web registration and transport becomes even more acute with the development of high-resolution non-contact printing, such as high-volume inkjet printing. With this type of printing system, finely controlled dots of ink are rapidly and accurately propelled from the printhead onto the surface of the moving media, with the web substrate often coursing past the printhead at speeds measured in hundreds of feet per minute. No impression roller is used; synchronization and timing are employed to determine the sequencing of colorant application to the moving media. With dot resolution of 600 dots-per-inch (DPI) and better, a high degree of registration accuracy is needed. During printing, variable amounts of ink may be applied to different portions of the rapidly moving web, with drying mechanisms typically employed after each printhead or bank of printheads. Variability in ink or other liquid amounts and types and in drying time can cause substrate stiffness and tension characteristics to vary dynamically over a range for different types of substrate, contributing to the overall complexity of the substrate handling and registration challenge.
One approach to the registration problem is to provide a print module that forces the web media along a tightly controlled print path. This is the approach that is exemplified in U.S. Patent Application No. 2009/0122126 entitled “Web Flow Path” by Ray et al. In such a system, there are multiple drive rollers that fix and constrain the web media position as it moves past one or more ink application printheads.
Problems with such a conventional approach include significant cost in design, assembly, and adjustment and alignment of web handling components along the media path. While such a conventional approach may allow some degree of modularity, it would be difficult and costly to expand or modify a system with this type of design. Each “module” for such a system would itself be a complete printing apparatus, or would require a complete, self-contained subassembly for paper transport, making it costly to modify or extend a printing operation, such as to add one or more additional colors or processing steps, for example.
Various approaches to web tracking are suitable for various printing technologies. For example, active alignment steering, as taught for an electrographic reproduction web (often referred to as a belt on which images are transported) in commonly assigned U.S. Pat. No. 4,572,417 entitled “Web Tracking Apparatus” to Joseph et al. would require multiple steering stations for continuous web printing, with accompanying synchronization control. It would be difficult and costly to employ such a solution with a print medium whose stiffness and tension vary during printing, as described above. Other solutions for web (or belt as referred to above) steering are similarly intended for endless webs in electrophotographic equipment but are not readily adaptable for use with paper media. Steering using a surface-contacting roller, useful for low-speed photographic printers and taught in commonly assigned U.S. Pat. No. 4,795,070 entitled “Web Tracking Apparatus” to Blanding et al. would be inappropriate for a surface that is variably wetted with ink and would also tend to introduce non-uniform tension in the cross-track direction. Other solutions taught for photographic media, such as those disclosed in commonly assigned U.S. Pat. No. 4,901,903 entitled “Web Guiding Apparatus” to Blanding are well suited to photographic media moving at slow to moderate speeds but are inappropriate for systems that need to accommodate a wide range of medias, each with different characteristics, and transport each media type at speeds of hundreds of feet per minute.
In order for high-speed non-contact printers to compete against earlier types of devices in the commercial printing market, the high cost of the web transport must be greatly reduced. There is a need for an adaptable non-contact printing system that can be fabricated and configured without the cost of significant down-time, complex adjustment, and constraint on web media materials and types.
One aspect of such a system relates to components that feed the continuous web substrate into the printing system and guide the web media into a suitable cross-track position for subsequent transport and printing. Conventional solutions for controlling the position of a moving web include approaches used for handling magnetic tape media used for data storage. For example, U.S. Pat. No. 3,443,273 entitled “Tape Handling Element” to Arch describes a roller mechanism that guides tape position by applying force that continuously aligns an edge of the moving tape with an edge-guiding cap on the roller; U.S. Pat. No. 3,850,358 entitled “Continuous Compliant Guide for Moving Web” to Nettles describes an arrangement of long, continuous compliant guides that register one or both sides of the moving magnetic tape; European Patent Application EP 0 491 475 entitled “Flexible Moving Web Guide” by Albrecht et al. describes a gimbaled compliant tape guide that employs a flanged roller for guiding the moving magnetic tape.
While conventional solutions such as these may work successfully for magnetic tape, however, these approaches fail to meet the needs of a print media handing system. Magnetic tape has a fixed size and confined stiffness range, unlike paper and other printing substrates, and magnetic tape thus presents a simpler mechanical task for maintaining constant tension and precise registration as it moves past read/write components. Close spacing between edge guides is possible with magnetic tape, allowing precise registration at high transport speeds; however, with paper and other print substrates, dimensional requirements make such tight control unworkable using closely spaced edge guides.
Conventional solutions for handling continuous web print media have also been found to be poorly suited for high-speed non-contact printing applications. For example, commonly assigned U.S. Pat. No. 5,397,289 entitled “Gimballed Roller for Web Material” to Entz et al. describes a gimbaled roller that positions itself automatically with respect to a moving web, but applies edge guidance along both edges, providing over-constraint not desirable for a kinematic web handling system. The '903 Blanding patent noted earlier describes the use of a compliant roller with a pivoted yoke and roller that urges an edge of the moving web of photographic print paper against an edge guide as it is fed from a supply roll. This type of solution works well for photographic paper, which has a relatively high cross-track stiffness and relatively narrow range of widths, but is not readily adaptable for print media that can be several times as wide as photographic print paper and, unlike photographic media, may have a broad range of stiffness and thickness characteristics.
The task of guiding a web into position within a printer has been traditionally done with a servo web guide or nipped edge guide assembly. Among problems with conventional web guides of these types are high parts count and assembly cost, complex mechanical constraint profiles, media handling problems due to localized nip pressure, and relatively high cost. Depending on the application, a traditional edge guide, such as those previously described in the literature, may have other shortcomings as well. Many conventional edge guide devices contact the top surface of the paper or other substrate with an “urging” roller that urges the paper against an edge guide. This can transmit a force through the paper onto the web support means, potentially damaging the web or smudging any colorant or other coating that may already be imprinted on the web surface. A conventional urging roller can also place a non-uniform drag on the paper due to a force imbalance between the edge and nip forces. It can also be difficult to accommodate large variations in paper width while maintaining center justification with this approach.
Among desirable characteristics of the input subsystem for web guidance are the following:                (i) accommodate a range of media widths and media having different stiffness, thickness, surface gloss, and other characteristics;        (ii) maintain center justification of the media web as it travels through the transport system; center justification is needed for kinematic web handling;        (iii) minimize parts count, mechanical complexity, and cost;        (iv) eliminate the need for an urging roller that applies force against the printed surface of the media web;        (v) eliminate point contact against the edge of the web;        (vi) able to accept input media from a slack loop, wherein the media upon input has very little cross-web stiffness, and to provide media being fed downstream, such as into a printing apparatus, with a higher amount of cross-web stiffness;        (vii) minimize mechanical constraint to the web as much as possible.        
Unfortunately, performance problems that may be inherent to various types of conventional web media edge guides and may not impact some types of systems become increasingly more pronounced as web transport speeds increase. While problems such as non-uniform drag and tendency to stray from center justification can be corrected to some degree with slower moving web transport systems, these problems are accentuated where high web transport speeds exceed 100 feet per minute. Difficulties of this type become even further complicated when system requirements allow for a range of media widths and types, having various stiffness, thickness, surface smoothness, and other characteristics, and when some of these characteristics can change dynamically, such as with the amount of applied ink or other fluids. There is, then, a need for a web edge guide that is suited to the demanding requirements of high-speed media transport for non-contact printing applications.